490 A MANUAL OF PHYSIOLOGY 



times accompanied by a slowing of the heart. A second 

 dose injected before the effect of the first has passed off is 

 inactive; and this distinguishes the pituitary from the supra- 

 renal extract. The depressor substance produces a marked 

 fall of blood-pressure, even when it is injected during the 

 rise of pressure caused by an injection of the pressor sub- 

 stance. 



Extracts of nervous tissue (sciatic nerve, white matter of 

 brain, spinal cord, but especially gray matter of brain) 

 cause, on injection into the veins, a decided fall of arterial 

 blood-pressure, which soon passes off, and can be renewed 

 by a fresh injection. The fall of pressure is due to direct 

 action upon the blood-vessels of a depressor substance in 

 the extracts, and not to the action of vaso-motor nerves. 

 It can be obtained after section of the vagi (Osborne and 

 Vincent). 



The removal of the thymus in the frog, in which animal the organ 

 persists throughout life,* is said to be fatal. The chief symptoms 

 are muscular weakness going on to paralysis, trophic disturbances, 

 including discolouration of the skin and certain alterations in the 

 blood (hydraemia, swelling of the red corpuscles and slight diminu- 

 tion in their number, with increase in the number of the white 

 corpuscles). The chief effect of intravenous injection of extract of 

 human or ox thymus is a lowering of blood-pressure. In this respect 

 it resembles thyroid extract. The heart may be at the same time 

 accelerated. 



The spleen does not produce an internal secretion necessary to- 

 life, for it can be removed both in animals and in man, not only 

 without causing death, but often without the development of any 

 serious symptoms. Its blood forming and blood-destroying functions 

 (p. 32) are taken on by other structures (particularly the red bone- 

 marrow), but the formation of the bile-pigment is interfered with, 

 and its amount reduced by more than 50 per cent. (Pugliese). The 

 production of trypsin by the pancreas is also said to be diminished, 

 whereas if an extract of spleen be injected into the circulation of an 

 animal deprived of its spleen, the amount of trypsin is increased. It 

 has, therefore, been supposed that the spleen forms a substance 

 (protrypsin) which, passing into the blood, is taken up by the 

 pancreas and elaborated into trypsinogen. 



The salivary glands may be extirpated without the slightest 

 change being produced in the normal metabolism. 



* In mammals (including man) the thymus does not completely dis- 

 appear in the adult. Islands of thymus tissue are found at all ages 

 among the fat by which the bulk of the organ is replaced. 



